1. Technical Field of the Invention
The present invention relates to a procedure and apparatus for touch-free examinations of objects, especially regarding their surface character.
2. Description of the Prior Art
Procedures and appliances, employing a touch-free technique to examine the surface character with the aid of optical resources are already existing. Surfaces may be illuminated using rays of light or oblique light to illuminate the surface from a flat or oblique angle, simply by localising and quantifying superjacent soil particles, unevenness, rough spots, processing traces etc. This relies on the fact that depending on the specification of such deviations compared to the target surface character, more or less intense bright/dark spots can build up i.e. due to illuminated and shaded edges, which in turn allows to draw conclusions about the three-dimensional surface character.
This proceeding is adequately known under the term ray of light or oblique light procedure, i.e. DE 197 16 264 A1. In reverse it is also used in a simplified manner for the production of topographic maps in order to provide a flat map with the impression of a plastic landscape with the help of an imaginary incidence of light. The desired effect is also known as shading or shadow plastic. Generally speaking there is a functional relationship between the angle of incidence, the three-dimensional orientation and the position of the sub-area, especially its inclination, height and the angle of emergence of reflected light. The reflected light is recorded as gray scale information with the means of an opto-electronic sensor primarily vertically inclined to the surface, i.e. a line or matrix sensor, and then transferred to digital image processing. This procedure is particularly well-suited for punctual, linear or frequently re-occuring surface deviations, such as soiling and scratches for example.
A strip light procedure projecting a geometrically defined pattern (for example bright and dark strips) with the help of a light source onto the surface of an object is also known. Depending on the specification of surface elevations and depressions, the projected pattern is deformed. For example, on a sub-area more exposed to the light source, strip width and distance are reduced. On the other hand, on a sub-area less exposed to the light source, i.e. a depression, width and distance are increased. A three-dimensional surface model may be derived from the pattern's deformation via suitable algorithms, after the reflected light has also been gathered with an opto-electronic sensor and its readings have been transferred to digital processing analysis. This procedure (also known from the DE 197 30 885 A1), which is also called coded light or projection procedure, is particularly well suited if a detailed surface structure with a corresponding light/dark contrast range is non-existent—for example on a smooth surface with large-area curvatures. Depending on the surface character, the emitted light of the light source is reflected from the surface by both the rays of light and the strip light procedure. The brightness and gray scale dispersion, which is being gathered by an image sensor, correlates more or less closely with this surface character.
However, these well-known procedures and appliances show a disadvantage: They can only supply three-dimensional surface features i.e. geometrical surface data of the object. Moreover the functional correlation between brightness dispersion and surface character only strictly apply to a surface with constant material qualities. The measurement result may turn out to be false when the surface combines various adjacent materials with different photometric or shape-independent features such as reflection, transmission and absorption parameters. This means that an overlap in the desired brightness dispersion of the above mentioned procedure occurs in case of surface character deviations with brightness data of multicolored, patterned or mottled surface segments that do not deviate from the target surface character, which complicates the assignment of the resulting brightness data to a distinct shape (for example an edge with an x incline) or a distinct material (for example soiling of the surface independent and material specific gray tone y).
The DE 198 39 882 includes a lighting system for color matching of coated car body surfaces intending to obtain surface features on the one hand and surface independent features on the other hand (for example material specific parameters such as chrominance errors) through differently arranged and implemented light sources. There is a differentiation between a basic lighting set, which hits the car body surface at a 45-degree angle of incidence and which is used as a working light for repairs and the detection of chrominance and polishing errors, and a structural light set with special implemented linear light sources that also emit at a 45 degree angle and helps to detect surface deformations, i.e. dents. Vehicles to be examined are directed through the large and stationary light set where mechanics do repair jobs and work on manual color matching of the car body surface. It becomes complicated, however, when a person needs to change the perspective in order to detect the different types of errors and is therefore no longer able to check one and the same surface segment. If the person wants to check the same surface segment for both types of errors he/she has to choose another position. Moreover this method only allows the assessment of either non-topographical types of errors (i.e. soiling or chrominance errors) or topographical types of errors (i.e. dents). A combined assessment is not possible. This may be compared to replacing visual observation with a camera a person is using instead. Altogether this procedure appears difficult and complex in its implementation and is only practical for glossy surfaces. The dimensions of the lighting set only facilitate its stationary operation.
In addition, the DE 35 40 228 A1 includes a description and a procedure for the implementation of controls for soldering points which works like this: From various angles of incidence, light is directed at the soldering points of a pcb and the reflected light from the soldering point is gathered by one or more B/W image sensors. This way, a majority of images with various informational content, depending on the angle of incidence, is generated. The accuracy of the soldering point structure may then be assessed with the help of a computational synthesis of the images. The reason for a decision (after the shape-independent characteristics have already been generated) whether (only if a distinct shape of the soldering point could not be assessed) there is a spot of minor solder, a circuit without solder or pcb material is the way in which the combination of shape characteristics or shape-independent characteristics is taking place—whereby the examination is always based on the same material.
A combined data processing for the reduction of shape-independent impacts of errors is not planned at this point. The arrangement itself consists of a stationary housing in which several light sources are set up to emit at various angles towards the soldering point. The housing is also equipped with a lighting aperture directed at the soldering point as well as a second aperture through which several cameras can gather the light reflected by the soldering point. The mobile pcb with soldering points arranged on the X/Y table is underneath the housing or its lighting aperture. This arrangement is disadvantageous as it is only used for stationary operation e.g. in a production line. Furthermore, just like with the housing's second aperture, the distance between the housing and the pcb is prone to extraneous light.